1. Field of Invention
This invention relates generally to the design of electrical machines, and more particularly to the design of alternators, generators and motors having low inductance in the armature circuits with particular emphasis on use in flywheel energy storage systems
2. Description of the Prior Art
As is well understood by those skilled in the art, electrical machines have an internal impedance that interacts with other system impedance to determine the performance of the combined system. In a motor, the inductance is that portion of the internal impedance related to magnetic energy storage within the electrical machine as it is energized to deliver mechanical work. The electrical system driving the motor must deliver the energy to be stored in the inductor in addition to the energy for the mechanical work to be performed. This necessitates increases in the capacity of generators, wiring and transformers needed to supply the motor.
In alternators and generators the internal impedance is, perhaps, even more important. The alternator or generator impedance combines with the load impedance to determine the performance of the whole system. As the internal impedance of an alternator or generator is made to be a smaller fraction of the total impedance, the output voltage of the alternator or generator becomes a larger fraction of the ideal (pre-loss) voltage provided by the idealized source. In the current art care is generally taken to provide low resistance pathways in the copper windings of an alternator or a generator in order to minimize internal resistance and to minimize the power lost in the alternator or the generator and the waste heat that needs to be dissipated.
Another factor in the impedance of the alternator or generator is the inductance of the output windings. This inductance is a direct result of winding the output coils around magnetic pathways in the alternator or generator, this being the technique usually used to generate the output voltage. Any output current in such windings will store magnetic energy in the same magnetic pathways, as is well understood. The inductance, "L" of the circuit is related to this stored energy by the equation EQU L=2*(Energy Stored)/(Current.sup.2)
The inductance of the output windings is part of the internal impedance and acts to filter the output voltage applied to the load. As frequencies get higher this inductive impedance blocks an increasing proportion of the ideal voltage provided by the alternator or generator and prevents it from acting on the load. While this has not been much of an issue for 60 Hz synchronous generators, it becomes a substantial design challenge for high frequency alternators. This has been known for some time; for example Griffing and Glockler present the design of a "High Frequency Low Inductance Generator" in U.S. Pat. No. 3,858,071.
High frequency alternators or generators are desirable in that high levels of output power can be achieved with physically small magnetic paths; resulting in physically compact units. Claw pole alternators are typical of the physical design of high frequency generator devices and achieve high frequency by having a plurality of alternating poles. A disadvantage of these physically small claw pole units is that the close proximity of multiple poles and multiple magnetic pathways allows for unnecessary storage of substantial amounts of magnetic energy, resulting in high output inductance.
High output inductance causes several difficulties in the operation of high frequency alternators or generators. The impedance, Z of the inductor grows directly with the operating frequency (co, rad/sec) as shown in the following formula: EQU Z(.omega.)=j*.omega.*L(j=imaginary operator)
The higher the frequency, the greater the impedance and filtering. To overcome this filtering, the ideal voltage must be increased as the frequency is increased. The ideal (pre-loss) voltage is usually increased by increasing the magnetic excitation level of the field, leading to higher magnetic intensity levels in the magnetic pathways. Since core losses due to eddy current generation are proportionate to both the frequency squared and the magnetic intensity level squared it will be understood that the need for extra excitation to overcome the inductive impedance of the output will lead to high core losses at high frequency operation. At the limit, when the excitation levels reach the point where magnetic pathways become saturated, further excitation is precluded, and the output of the device drops off with further increases in operating frequency.
As a counterpoint to this, if internal inductance were negligible, then the output voltage would rise with increasing frequency due to the increased change of flux with time . The excitation levels could then be reduced as the frequency increased, leading the device away from saturation. The decrease in excitation would offset the increase in frequency such that the core losses would remain nearly constant with operating frequency.
It is therefore an objective of this invention to provide an electrical machine which may be used as a high frequency alternator with low output inductance.
Furthermore, high frequency alternators are often poly-phased devices used with solid state circuits to rectify, switch, commutate or chop the output and reform it into DC or desired power frequency (50 or 60 Hz, etc) AC forms. In such devices individual alternator output phases are turned on and off at high frequencies, again invoking the filtering of the output inductance. Also, it is common for the output inductance of one phase to be linked by mutual inductance to the output of other phases so that the sudden change in current (switching) in one phase produces unwanted voltage transients in the other phases.
It is therefore a further objective of this invention to provide an electrical machine which may be used as a poly-phase high frequency alternator with minimal adverse effects caused by mutual inductance between phases.
As noted for the typical high frequency alternators, such as the claw pole type, the close proximity of multiple poles and magnetic paths gives rise to the unnecessary storage of large amounts of magnetic energy. This is important in the field excitation circuit as well as the output circuit because of the saturation and core-loss issues already mentioned. It should be noted that the majority of magnetic energy is stored in the high reluctance air spaces which are interconnected by the low reluctance ferro-magnetic pathways in which the saturation and core-losses phenomenon occur. So called "leakage flux" passes through the air spaces to complete magnetic circuits without going through the intended pathways which link output coils. In physically compact machines where many poles and pathways are arranged in close proximity the leakage flux can become a high percentage of flux, making the machine inefficient.
Alternators and generators based on armatures with poloidal windings around a stator shaped as an annular ring (or toroid) have long been know. Kirkley and Smith present a design based on radial air gaps in U.S. Pat. No. 4,087,711. Further improvements were presented by Radovsky in U.S. Pat. No. 5,798,594 in which a brushless synchronous machine is presented with axial air gaps completing the magnetic circuit through an annular ring stator in a fashion that greatly limits the leakage flux from the field. The rotors in these designs are relatively complex, present difficulties in establishment of air gap clearances, and do not address the issues of output inductance or mutual inductance between phases.
It is therefore a further objective of this invention to provide a low inductance electric machine which may be used as a brushless alternator with low leakage of the field flux combined with simplified rotor construction and provision for independently establishing multiple air gaps.
Recent work by Groehl, disclosed in U.S. Pat. No. 5,565,836, presents methods for achieving the nullification of unnecessary components of flux within a toroidally wound inductor by use of concentric windings around the arcuate axis of the toroid combined with electrical connection of such windings so that currents of equal magnitude and opposite direction provide flux cancellation. While the cancellation of internal flux fields within the toroid can be used to create low inductance, and is fundamental to the design and operation of the common electrical power transformer, the heretofore known configurations have not been suitable for the efficient transformation between mechanical and electrical energy.
It is therefore a further objective of this invention to provide an electrical machine wherein the benefits of flux cancellation within a toroid, or other closed shape, can be combined with arrangements for the efficient transformation between mechanical and electrical energy.
The previously mentioned high frequency alternators may be used for the production of (low) power frequency (60 Hz) AC power through methods disclosed by Hilgendorf in U.S. Pat. No. 3,916,284 and improvements presented by Tupper in US patent application Ser. No. 09/002,121, "Apparatus for Resonant Excitation of High Frequency Alternator Field". In this use the field excitation of the high frequency alternator is subjected to 60 Hz amplitude modulation. This leads to 60 Hz fluctuations of the magnetic field throughout the alternator's magnetic core, with attendant possibilities for eddy current core losses. Many traditional alternators are designed for essentially constant levels of magnetic excitation. These traditional alternators typically use core structures, such as solid iron rotors, that are not optimized to reduce eddy current losses. For constant levels of excitation this is acceptable as there is little change in field excitation and therefore little core loss. These traditional alternators are not suited to use with 60 Hz amplitude modulation of the field; the core losses due to field modulation would be too large.
It is therefore a further objective of this invention to provide an electrical machine wherein the field excitation may be amplitude modulated at power frequencies while core losses are minimized.
As is well known, many electromechanical devices can be run in either a motor or generator mode. A generator with low internal inductance might also be operated as a motor with low internal inductance. In motor operation, low internal inductance reduces the requirements for the electric supply system to handle energy which is stored in the magnetic field of the device. Furthermore high armature inductance can impede the rapid change of armature pole currents and magnetic fields, thereby restricting the high frequency response of the motor. A motor with low armature inductance would allow relatively high frequency operation of the motor, which is useful in variable speed applications.
It is therefore a further objective of this invention to provide an electric machine which may be used as a motor with low inductance of the armature circuits.
In stepper motor operations precise control of the shaft position is achieved through the creation of a discrete step relationship between rotor pole position and armature excitation. This is useful for many industrial applications requiring careful control of shaft position. It is therefore a further objective of this invention to provide for an electrical machine with low armature inductance and which may be operated like a stepper motor.
In contrast to the stepper motor operations, during synchronous motor operations it is desirable to achieve a smooth rotation of the shaft. It is an additional objective of this invention to provide a low inductance electric machine that may be used as a synchronous motor with smooth rotation of the shaft.
Where motors are used in systems with requirements for regenerative braking it is desirable to be able to control the regenerated voltage and current independently from shaft RPM. For example, an electrically powered automobile using regenerative braking to stop at a traffic light would need to control the generated power so that it was constantly suitable for recharging the vehicle battery even as the vehicle slows to a stop.
It is a further object of this invention to provide an electromechanical device that can be operated as a motor and which can be switched to operation as a generator with controllable output voltage to extract electrical energy from the momentum of the driven system.
Rapidly rotating flywheels may be used to store substantial amounts of mechanical energy. Electrical machines which can efficiently act as motors and generators at the flywheel rotor speeds of 20,000-100,000 RPM are an essential part of flywheel energy storage systems. In the present state of the art permanent magnet motor/generators are built wherein the rotors act both as electromechanical conversion devices and as flywheels for the mechanical storage of energy. The device is operated as a motor with no external load in order to speed up the flywheel to store more energy. The device is operated as a generator, slowing the flywheel rotor's spin rate as energy is withdrawn. Such devices are proposed for storing and delivering "peaking" power in systems that use solar panels or small internal combustion engines to provide the average power over time but where these average power sources are not designed to handle peak power loads. Permanent magnet devices have output voltages that vary with the shaft speed so cumbersome power conditioning equipment is needed to maintain a steady output voltage. Since the flywheel RPM varies with the amount of energy stored it is desirable to have a device with controllable voltage output regardless of flywheel shaft speed.
It is a further object of this invention to provide an electro-mechanical device that can be operated as a motor to store flywheel energy in its rotor, and which can be operated as a generator with controllable output voltage to extract electrical energy from the flywheel energy of its rotor.
Flywheels have large gyroscopic effects which can be problematical for use in terrestrial and space vehicles. One solution is to have two flywheel energy storage devices rotating in opposite directions so that the gyroscopic effects can be made to cancel. This generally means at least two separate devices connected so that mechanical reaction forces can cancel each other, and a control system to keep the rotor speeds in a desired relationship (usually equal). U.S. Pat. No. 5,124,605 by Bitterly, et al. presents a flywheel energy storage device with counter-rotating flywheels integral to a single device.
It is an additional object of this device to provide a low inductance electro-mechanical device for use as a flywheel energy storage device wherein the gyroscopic effects are nullified within a single device.